And how would they do that if your emails are encrypted?
They (probably) can't. So they can speak to your email RECIPIENT (if in the UK) and wave a RIPA document. You would never know.
Anyway, I think that you are missing the point.
Data about the recipient's address is just as valuable as the content.
Your network of friends, THEIR networks of friends will tell a lot about YOU.
Maybe not that you are about to raid a beagle lab at 13:07 on the 17th ... but that you an an animal rights associate.
When 50 homes are raided your's will be too.
Ever hear about Murder Inc in the USA?
Phone call records (not taps) were used to build up a picture of the two gangs concerned.
One key suspect turned out to be ... an innocent pizza house used by many of the gangsters!
Nevertheless many previously unknown West Coast Mafia members and the New York contract killers they used were identified.
If that can be done with called number records, just think of what can be found when you merge landline call records, mobile phone call records, credit card data, direct debit data, credit check data, email records, website visit records, car number plate recognition records, medical records, insurance records ... oh, yes .... and forum post data.
Privacy is a thing of the past.